[rs911t]

Thanks all for the ideas.

Matt, the car in question is an 80 930 race car. The backfiring was sporadic,
as was the noise. I didn't notice that the idle decreased.
It continued to idle and otherwise sound fine, which the occasional exhaust
backfire. Similar to what the 2.8 does when it has a clogged idle jet.

I inspected the rockers visually for the obvious. All we in place and had
their adjustment nuts tight. Also wiggled each as I did the leakdown test
to check for unsually free movement.

I'll go back thru and check the gaps, feet and springs.

More info to come! Thanks ...

[Don Wohlfarth]

Matt, Greg's race car is a modified 930 and I think with stock CIS.
Of course if Greg is running 11 to 1 cr that could explain the problem.

[rs911t]

Hmm ... 11:1 compression ... 1.3 bar boost ... engine that doesn't explode ...

That could be fun!

[Charlie Stylianos]

I like Matt's suggestion of checking the chain tensioners. If it's intermittant, the tensioners could be on their way out.....which would also explain a backfire when you hear the sound (valve timing slightly out of wack).

Was it a 'chain dragging through a metal trash can' type sound, or more rythmic, like a tick-tick?

You can pull the chain covers off with a little work and look for fresh marring on the chain box from a chain dragging. I'd also check any oil lines running to the hydraulic tensioners (assuming you have that type) and make sure they are not pinched, cracked or leaking at the joints.

[APKhaos]

Its definitely a repetitive tick-tick, at about 1 tick per 2 revs [which is what made me thing valve gear in the first place]. The miss is solid at idle - one pot not firing. The backfire must have been on track - I didn't hear it in the paddock.

Taking a borescope to have a look at the top of the pistons sounds a good next step in the non-destructive test plan.

Tony, have you figured out which cyl is missing at idle? If not, start pulling plugs until you get to one that doesn't cause a change in the miss. This will be the cyl that is affected. Then again, maybe you've already narrowed it down.

[Vicegrip]

Greg, when do you want to get together and scope the motor? Wed night looks good for me.

[rs911t]

Kurt, Wed could work. I'll give you a call late in the day. Thanks.

[William Miller]

With that CR and hot cams tolerances are probably very small. Cam timing very important in making sure nothing touches.

[Vicegrip]

Hmm... "Tapping" sound, backfire, no loss of idle. A leak at the head/header gasket could cause this as well.